Can an Airplane Take Off on a Moving Runway?

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Can an Airplane Take Off on a Moving Runway?

This question is probably as old as the airplane itself. It goes something like this:

An airplane has a takeoff speed of 100 mph (I just made that number up). What if it gets on a super giant treadmill that moves backwards at 100 mph. Could a plane on this giant treadmill take off or would it just sit there moving at 0 mph?

The first question a reasonable person would ask is "Where do you get a giant plane-sized treadmill that goes 100 mph?" Yes, that is indeed a good question—but I won't answer it. Instead, I'm going to give this question the best physics answer I can.

Before I do that, I should point out that others have also answered this question (not surprising since it's super old anyway). First, there is the MythBusters episode from 2008. Actually, they didn't answer the question—they did the question. The MythBusters made a giant conveyer belt with a plane on it. It was awesome. Second, there is the xkcd answer to this question (also from 2008).

Now you get my answer. I will answer with different examples.

A Car on a Conveyer Belt

This isn't so difficult. What if I put a car going 100 mph on a conveyer belt that is also going 100 mph? It would look like this (something like this):

Really, there is probably no surprise here. The car's wheels would roll at 100 mph as the treadmill (or conveyer belt) moves back at 100 mph so that the car remains stationary. Actually, here is a slightly cooler example (with the same physics).

Here is an experiment (also from the MythBusters) in which they shot a ball at 60 mph out the back of a truck also going 60 mph. You can see that the ball remains stationary (with respect to the ground).

Super Short Takeoff

Here is a plane from Alaska that takes off in a very short distance.

How does this work? I'll give you a hint—there is a very strong wind blowing into the front of the plane. Without a headwind, this wouldn't happen. But if you think about it, this short take off is very much like the car on the treadmill. For a plane, it doesn't drive on the ground, it "drives" in the air. If the plane has a takeoff speed of 40 mph and is in a 40 mph headwind, it doesn't even need to move at all with respect to the ground.

Plane on a Conveyer Belt

Now let's do it. Here is a short clip from the MythBusters launching a plane on a moving treadmill.

Yes, it takes off. A plane can take off from a runway moving in the opposite direction? But why? It's because the wheels on a plane don't really do anything. The only function for the wheels is to produce low friction between the aircraft and the ground. They don't even push the plane forward—that is done by the propeller. The only difference when launching a plane on a moving runway is that the wheels will spin at twice the normal speed—but that shouldn't matter.

So the plane on a treadmill works, but how about a case where the plane wouldn't take off? What if the plane was more like a glider with motorized wheels? On a normal runway, these motorized wheels would increase the speed of the glider until it reached takeoff speed. But if you put this on a moving runway, the wheels would spin at the right speed and cancel the motion of the treadmill so that the plane would remain motionless and never reach the proper speed for a launch.

OK, so that is the answer to everyone's favorite question. But don't worry, this answer won't stop the endless discussion—that will live on forever.

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